Nitrogen Loading from Onsite Wastewater Treatment Systems in the Greater Narragansett Bay (Rhode Island, USA) Watershed: Magnitude and Reduction Strategies

Onsite wastewater treatment systems (OWTS) are an important part of the water infrastructure in the USA. Advanced OWTS are used instead of conventional OWTS to lower nitrogen (N) inputs to coastal ecosystems and groundwater sources used for drinking. Knowledge of the N load from OWTS helps identify drivers of excess N and develop strategies to lower N inputs. We used wastewater flow and effluent total N (TN) concentration to determine the mass N load from 42 advanced N-removal OWTS technologies (Orenco Advantex AX-20®, BioMicrobics MicroFAST®, SeptiTech D® series) and 5 conventional OWTS within the Rhode Island, USA, part of the Greater Narragansett Bay watershed. The median N load (g N/system/day) followed the order: conventional systems (31.1) > AX-20 (10.8) > FAST (10.1) > SeptiTech (9.6), and was positively correlated with flow. Results of a Monte Carlo simulation estimated the N load from the current distribution of conventional and advanced systems (105,833 systems total; Current scenario) to the watershed at 1,217,539 kg N/year. Compared to the Worse Case scenario (100% conventional OWTS), advanced OWTS currently prevent 53,898 kg N/year from entering the watershed. The per capita N load (kg N/capita/year) from OWTS under the current scenario is 4.68, and 1.47 for a local wastewater treatment plant (WTP) with biological N removal (BNR). Replacing 5150 conventional OWTS yearly with the most effective OWTS technology would result in a per capita N load from OWTS equivalent to that for a WTP with BNR after ~15 years, with a yearly cost of $174.24 per additional kilogram of N removed. Increasing the proportion of advanced OWTS that achieve the final effluent standard of 19 mg TN/L—through monitoring and recursive adjustment—would reduce the time and cost necessary to achieve parity with the WTP. Advanced N-removal OWTS are an important part of the water infrastructure that can lower N load to the Narragansett Bay watershed.